Polarization multiplexers and demultiplexers are capable of providing a variety of functions in lightwave telecommunications and signal processing systems. These devices permit signal combining and signal splitting or signal tapping for feedback controlled polarization stabilization or even simply signal polarization for fiber gyroscopic applications.
Passive polarization multiplexers and demultiplexers have been demonstrated using dielectric waveguides which intersect or form a Y branch. See, for example, M. Masuda et al., Appl. Phys. Lett., Vol. 37, pp. 20-22 (1980) and H. Nakajima et al., IEEE Trans. MTT, MTT-30, pp. 617-621 (1982). On the other hand, a more interesting and versatile class of these devices includes active polarization multiplexers and demultiplexers. See, for example, U.S. Pat. No. 4,390,236 issued to R. C. Alferness on June 28, 1983 and O. Mikami, Appl. Phys. Lett., Vol. 36, pp. 491-3 (1980).
Active polarization multiplexers and demultiplexers utilize dielectric waveguides in the electrooptic directional coupler architecture. Electrodes directly over or adjacent to the waveguides control switching of the directional coupler for either the straight-through state or the crossover state. Parameters of the directional coupler are varied in order to allow the device to respond properly to particular polarizations of input optical signals, that is, the transverse electric (TE) mode or the transverse magnetic (TM) mode. Both of the active devices cited above incorporate phase mismatching of the waveguides for one of the propagating polarizations. Phase mismatch results from additional processing during device fabrication as described below.
For the device shown in the Mikami article cited above, a single metallic electrode is in direct contact with one waveguide while a split metallic electrode is separated from the other waveguide by an intermediate buffer layer. This device utilizes metallic loading of one waveguide to reduce the propagation constant. Unfortunately, metallic loading of a waveguide simultaneously increases the attenuation constant of the TM mode in the metallically loaded waveguide. As a result of the metallic loading, which is essential for polarization selectivity, low propagation loss for the device is possible only when the input light signal is incident on the waveguide which is buffered from the metallic electrode. Therefore, this device is not suitable for polarization-independent applications which require that both polarizations (TE and TM) be coupled into each waveguide.
The polarization selective coupler described in the Alferness patent (his FIG. 2) incorporates the directional coupler architecture mentioned above. However, an intermediate buffer layer is situated between the set of electrodes and the waveguides. Also, Alferness changes the extraordinary refractive index of only one waveguide with respect to the other guide by employing special waveguide fabrication techniques This, in turn, causes the difference in phase propagation constants for one polarization (TM in z-cut lithium niobate) to be significantly greater than zero, while the difference in propagation constants for the other polarization (TE in z-cut lithium niobate) remains essentially zero. Although this coupler permits optical signals of each polarization to be introduced into either waveguide without any of the deleterious effects presented by the device of Mikami, it should be noted that the multiple step waveguide fabrication procedure adds a significant degree of complexity to the overall device fabrication procedure.